Field of the Invention
The present invention relates to a fixing device and an image forming apparatus.
Description of Related Art
A fixing device is provided in an image forming apparatuses such as a printer, a copier, a facsimile machine and a multifunctional peripheral including these apparatuses. A fixing device fixes a toner image which has been formed on a recording sheet based on input image data. In a fixing device, a recording sheet on which a toner image has been formed is pressed and heated when the sheet passes through a fixing nip portion formed between a heating member (for example, a fixing roller and a fixing belt) and a pressing member (for example, a pressure roller), and thus a toner image is fixed on the recording sheet.
Conventionally, there has been a problem that image quality and sheet feeding performance are degraded by factors, which are caused at the time of the fixing, such as an offset in which toner adheres to the surface of the fixing roller, and winding in which a recording sheet on which a toner image has been formed winds around the fixing roller. To solve such a problem, a technique has been proposed in which a wax-added toner is used so that wax eluted from toner dissolved at the time of heating is supplied to the fixing roller, and thus the offset and the winding of the recording sheet are suppressed.
However, in the case where part of the wax supplied to the fixing roller from the toner adheres to the fixing roller and a latent image due to the wax is formed on the fixing roller, the amount of the wax existing on the image surface varies at the time of the next fixing. As a result, there has been a problem that a phenomenon such as uneven gloss (hereinafter referred to also as “gloss memory”) which occurs due to the latent image on the fixing roller, thus degrading image quality. In order to prevent the gloss memory from occurring, it has been necessary to increase a pressing load at the fixing nip portion, in other words, a pressing load between the fixing roller and the pressure roller, by, for example, about 1.5 times as compared to a conventional pressing load.
It is to be noted that, as a technique relating to a fixing device, a technique is proposed in which an induction heating section is displaced in association with displacement of a heat generation member in a fixing device of an electromagnetic induction heating type (see, for example, Japanese Patent No. 4841179). According to the technology disclosed in Japanese Patent No. 4841179, even when components of the fixing device are thermally deformed, variation in the positional relationship between the induction heating section and the heat generation member is small, and thus defects such as variation in heat generation efficiency can be suppressed.
In addition, a technique is proposed in which the posture of a recording sheet being advanced to a pressing zone is changed to a favorable posture according to the basis weight of the recording sheet, thereby suppressing formation of dents and scars on a resin layer on the surface of a fixing member (see, for example, Japanese Patent No. 4821594).
However, there is a problem that, when a great pressing load is exerted by the heating member and the pressing member in order to prevent the uneven gloss from occurring, waves 20 (hereinafter referred to as “wavy wrinkles”) are formed along the conveying direction on recording sheet 10 as illustrated in FIG. 7A as a side effect.
Wavy wrinkles 20 are considered to be formed by the following mechanism. FIG. 7B illustrates a configuration of a fixing device. The fixing device includes a heating roller (not shown), fixing roller 40, endless fixing belt 50, and pressure roller 60. Fixing roller 40 includes elastic layer (for example, a rubber layer) 30. The heating roller and fixing roller 40 are spaced a predetermined distance apart from each other. Fixing belt 50 is provided around the heating roller and fixing roller 40. The pressure roller 60 is disposed in a state where it is in pressure contact with the fixing belt 50 in a region where the fixing belt 50 and fixing roller 40 are in contact with each other. Fixing nip portion 70 having an upwardly-raised form is formed at a part where the fixing belt 50 and pressure roller 60 make contact with each other.
In fixing nip portion 70, pressure roller 60 presses elastic layer 30 of fixing roller 40 to dent elastic layer 30. At a portion around an end portion of fixing nip portion 70 on the entrance side, portion 80 (hereinafter referred to also as “bulging portion”) where deflection of elastic layer 30 caused by the pressure contact of pressure roller 60 has a downwardly-raised form is formed. Along with the rotation of fixing roller 40 and pressure roller 60, bulging portion 80 of elastic layer 30 expandingly and contractingly deforms in an axial direction of fixing roller 40. As a result, in fixing belt 50, a portion in contact with bulging portion 80 of elastic layer 30 is slightly undulated, and thus wavy wrinkles 20 are formed on recording sheet 10 which has passed through fixing nip portion 70 as being sandwiched between fixing belt 50 and pressure roller 60.